properly detect ssa form

Previously, we would skip ssa construction when
a temporary has a single definition.  This is
only part of the ssa invariant: we must also
check that all uses are dominated by the single
definition.  The new code does this.

In fact, qbe does not store all the dominators
for a block, so instead of walking the idom
linked list we use a rough heuristic and declare
conservatively that B0 dominates B1 when one of
the two conditions is true:

  a. B0 is the start block
  b. B0 is B1

Some measurements on a big file from Michael
Forney show that the code is still as fast as
before this patch.

all.h

@@ -248,7 +248,7 @@ struct Use {
 		UIns,
 		UJmp,
 	} type;
-	int bid;
+	uint bid;
 	union {
 		Ins *ins;
 		Phi *phi;
@@ -279,6 +279,7 @@ struct Alias {
 
 struct Tmp {
 	char name[NString];
+	uint bid; /* id of a defining block */
 	Use *use;
 	uint ndef, nuse;
 	uint cost;